Satellite radio operators will soon provide digital quality radio broadcast services covering the entire continental United States. These services will offer approximately 100 channels, of which approximately 50 channels in a typical configuration will provide music with the remaining stations offering news, sports, talk and data channels. Digital radio may also be available in the near future from conventional analog radio broadcasters that will provide a terrestrial based system using signals colocated in the AM and FM bands.
Satellite radio has the ability to improve terrestrial radio's potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHZ) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by Sirius Satellite Radio and 12.5 MHz of which are owned by the assignee of the present application “XM Satellite Radio Inc.”
The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation of multipath interference and addresses reception issues associated with blockage of the satellite signals.
In accordance with the XM frequency plan, each of two geostationary Hughes 702 satellites will transmit identical or at least similar program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite 1 and satellite 2) will be time interleaved to lower the short-term time correlation and to maximize the robustness of the signal. For reliable reception, the LOS signals transmitted from satellite 1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by terrestrial repeaters. The XM System supports around 50 channels of music or data in Time Division Multiplex (TDM) format.
In any event, each of the digital audio radio systems described above will likely be received by a vehicle radio and powered by a primary battery that operates the vehicle itself. Industry leaders in car radio electronics will be offering 3-band AM/FM/XM radios that will be manufactured to replace existing car radios or will provide factory-installed AM/FM/XM radios to the automobile industry. Additionally, some of these radios may have the capability to provide data applications and even some telematics functions. These radios may work fine in operating the digital audio radio when the vehicle is operating to listen to a live broadcast or to perform data functions. Unfortunately, to adequately download relatively large amounts of data to the receiver when the vehicle is not operating will draw current from the vehicle battery in excess of automobile manufacturer specifications. For example, if a digital audio radio system contemplates downloading daily news files that requires the receiver to be powered for several hours a day for downloading such files, a vehicle battery could drain below specifications if the vehicle is being operated. Thus, a need exists for a method and apparatus for backup power in a communication system.